The present invention relates to a zinc-cobalt alloy plating alkaline bath suitable for forming a zinc-cobalt plating layer on a substrate such as parts for automobiles and a method for forming a zinc-cobalt plating layer using the plating bath.
The zinc-cobalt alloy-plating bath is attracting much attention because the bath can provide a plating layer having excellent corrosion resistance at a very low rate of cobalt-eutectoid. Japanese Un-examined Patent Publication No. Hei 2-282493 discloses a zinc-cobalt alloy-electroplating alkaline bath comprising a zinc compound, a cobalt compound, an alkali hydroxide, a chelating compound and a brightener. However, this plating bath suffers from a problem in that the amount of the chelating agent must be increased in order to achieve a rate of cobalt-eutectoid required for forming a plating film showing high corrosion resistance through the use of such a plating bath. Moreover, the plating bath also suffers from other problems of, for instance, operating flexibility, bath control, waste water disposal or treatment and cost for plating.
For this reason, there has been desired for the development of a zinc-cobalt alloy-plating bath and a method for plating a zinc-cobalt alloy layer on parts such as those for automobiles, which permits the solution of the foregoing problems associated with the conventional technique.